1. Field of the Invention
The invention relates to a signal processing technology. In particular, the invention relates to a method of detecting an error included in a data signal, an error detection circuit using the method, an error correction circuit using the method, and a reproducing apparatus using the method.
2. Description of the Related Art
High-capacity optical disk devices using red laser, such as a digital versatile disk (DVD), have been already put to practical use. Recently, next-generation optical disks which use blue laser sources of near 650 nm in wavelength are under intensive development for the sake of further enhanced recording capacities. Maximizing the recording capacities requires that higher error correction capabilities be provided aside from improved linear recording densities and narrower tracks resulting from a reduction in the wavelength of the recording light sources.
For example, DVD disks have minimum mark lengths of 0.6 micrometers and above. Next-generation optical disks using a blue laser source have minimum mark lengths of no greater than 0.3 micrometers. With the decreasing minimum mark length, the amount of information capable of being written to an identical area physically increases. Thus, given the same dimensions of flaws or dust particles on a disk surface, burst errors occurring on a next-generation disk have impact twice or more higher than on a DVD disk. There have thus been some techniques in which error correction code for burst error detection (hereinafter, referred to simply as “burst indicator subcode” or “BIS code”) is distributed within data in a discrete fashion and is used for correction (for example, Japanese Patent Laid-Open Publication No. 2001-515641).
FIG. 1 is a diagram showing the distribution of errors in a code block 10. In this diagram, the vertical direction is the encoding direction, and the horizontal direction is the recording direction and the reading direction of an optical disk. The code block 10 contains BIS code 20, user data 12 which is encoded in LDC (Long Distance Code), and LDC parities 24. The BIS code 20 includes ID information 14, reserve information 16, and BIS parities 18. For example, 216 bytes of user data 12 are accompanied with 32 bytes of parity 216, and 30 bytes of BIS code 20 are accompanied with 32 bytes of parity. In this diagram, “X” represents a location where an error occurs. Random errors 30 which occur sporadically and burst errors 36 which occur continuously are shown separately. A first burst error 36a is a code string that can be corrected by erasure correction. A second burst error 36b is a code string that cannot be detected as a burst error and thus is incapable of erasure correction.
Burst errors occur from such factors as disk flaws and adhesion of dust particles to the disk surface. When these phenomena arise, a large change typically occurs in the intensity of the reflected laser beam. A signal reproducing circuit amplifies the output of an intensity detector typically mounted on the head by using a head amplifier. The resultant is passed through a variable gain amplifier called AGC and/or an equalizer circuit for RF waveform shaping, followed by binarization and demodulation. Here, if the input signal level exhibits a large transient or the output amplitude of the head amplifier drops significantly, the binary output becomes “high” or “low” in level so that the demodulator often outputs 0s or 1s consecutively.
Random errors 30 are detected and corrected by using the LDC parities 24. Burst errors 36 are subjected to erasure correction. To perform erasure correction, it is necessary to recognize the locations of the burst errors 36. The BIS code 20 is used for this purpose. The BIS code 20 is arranged at predetermined logical addresses in stripes on the code block. To estimate the location of a burst error 36, error correction on the BIS code 20 is initially performed by using the BIS parities 18, so that error locations on the BIS code 20 are identified. Since the BIS code 20 is arranged at the predetermined logical addresses in stripes, a burst error 36 is estimated to occur near a location where errors of the BIS code 20 are corrected consecutively in the recording direction. Then, erasure correction is performed with that location as an erasure location.
In general, the reserve information 16 has bit values of “0” while a burst error 36 results from a series of predetermined bit values of either “1” or “0”. Thus, even if a burst error having bit values of “0” occurs across a piece of reserve information 16, that location cannot be corrected by the correction of the BIS code 20 since the reserve information 16 is not erroneous in terms of the bit value itself. In the diagram, first symbols 32 marked with “O ” on “X” represent the locations where error correction is conducted by using the BIS parities 18. A second symbol 34 marked with “O ” indicates that the location, despite being a burst error, will not be recognized as an error. For erasure correction, erasure correction locations 22 are identified by using the error-corrected locations of the BIS code 20 as clues. Thus, a location where no error correction is performed on the BIS code 20, i.e., where the user data 12 and the reserve information 16 match with each other and look like having no error, such as shown by the second symbol 34, will not be detected as an error location despite of the burst error 36. It is therefore impossible to estimate the erasure location of the second burst error 36b, or perform erasure correction thereon. In this way, there can often occur such cases that second burst errors 36b cannot be detected.